The present invention relates to a method of rotating a sheet advanced in a straight line in the direction of the advance, in a transport plane, by transport means, by retaining the sheet at a center of rotation. A method of this kind is known from U.S. Pat. No. 4,445,679 which describes a method in which during retention of an advancing sheet at the center of rotation, a slip occurs between the sheet transport means and the sheet. The sheet transport means must be able to allow such a slip, e.g. by making the transport means relatively smooth and/or by pressing it against the sheet with a relatively minor force. In these conditions there is the risk that when a sheet which has a relatively smooth surface is rotated, the sheet will slip as a whole so that the required rotation becomes incomplete. In the case of the rotation of a relatively thin sheet and/or a sheet with a rough surface, the sheet will not slip but will tear or crease.
Accordingly, the object of the present invention is to provide a method and apparatus which does not have these disadvantages.
The present invention is characterized in that during the rotation of the paper, the transport means forms solely a first transport nip which, when considered in a direction transverse to the direction in which the sheet is advanced in a straight line, is situated at a fixed distance from the center of rotation on a line between the first transport nip and said center of rotation which line includes an angle of between 70 and 90.degree. with respect to the direction of the advancing sheet. Consequently, a sheet can be reliably rotated in a simple manner with the sheet transport means operating continuously. Preferably, the angle is between 75 and 85.degree.. Consequently, the frictional force exerted by the sheet transport means on the sheet during the rotation thereof has a small component which keeps the sheet taut between the center of rotation and the first transport nip so that creasing of the paper is advantageously avoided, but without said small component becoming so large that the sheet or the image thereon is damaged.
Furthermore, preferably and prior to and subsequent to the execution of a rotary movement, the sheet is also advanced through a second transport nip which, when considered in the direction in which the sheet advances in a straight line, is in an identical position to the first transport nip. This minimizes the risk of skewing before and after rotation.
The apparatus for performing the present method comprises retaining means movable between a first position which are free of an advancing sheet and a second position in which they retain the sheet at the center of rotation, and is characterized in that coupling means are provided between the retaining means and the sheet transport means. Upon movement of the retaining means from the first position to the second position, the sheet transport means moves from a position in which the second transport nip is closed to a position in which the second transport nip is open. This is a simple construction to ensure that no slipping of the transport nip occurs during the rotation of the sheet of paper.
If the first and second transport nips are formed by two pressure rollers fixed on a common shaft and if the shaft is lifted at the second transport nip when the second transport nip is opened, the pressure roller is shifted upwards from the first transport nip to form a punctiform first transport nip which further reduces the slip in the first transport nip.
In an attractive embodiment of the apparatus according to the present invention, the first transport nip is situated at a predetermined distance from an upstream sheet feed nip which is smaller than the length of the shortest sheet for processing and a sheet discharge nip situated downstream of the first transport nip is located at a predetermined distance from the sheet feed nip which is somewhat greater than the length of the diagonal of the largest sheet for rotation. Consequently, a sheet is rotated about the center of rotation a short distance from the original leading edge and the sheet edge, which becomes the leading edge after the rotation of a quarter revolution is situated at a greater distance downstream of the center of rotation so that the sheet can be discharged relatively rapidly from the rotational zone.